1. Field of the Invention
The present invention relates to a sheet humidification apparatus that humidifies a sheet on which an image is formed, a sheet humidification method and an image formation system.
2. Description of Related Art
In electrophotographic image forming apparatuses such as a copier, a printer, a facsimile machine, and multifunctional devices having such functions, thermal fixation processes are performed to stabilize the toner image on the sheet. In the fixation processes, the moisture of the sheet is reduced.
As a result, the sheet may be curled or deformed into a wavy shape. In addition to the curl, the sheet may also have static electricity, or heat in the fixation processes. Consequently, jam may be easily caused, and the subsequent processes may be negatively influenced.
To solve the above-mentioned problems, a configuration for humidifying the sheet to be subjected to the fixation process has been proposed. Specifically, for the purpose of removing the curl of the sheet, discharging the sheet, and cooling down the sheet, the sheet is conveyed in a sandwiching manner with moisturized humidification rollers to humidify the sheet. In addition to the configuration using the humidification roller, a method of spraying water vapor to the sheet has also been proposed.
Variations and approaches of such image forming apparatuses are disclosed in Japanese Patent Application Laid-Open No. 2014-091605 and Japanese Patent Application Laid-Open No. 2012-024953.
Japanese Patent Application Laid-Open No. 2014-091605 discloses a mechanism for humidifying sheets during horizontal conveyance in an image formation system in which sheets are conveyed in the horizontal direction. Here, the distance between a water supply section and a humidification roller is set to the same distance between upper and lower humidification rollers to equalize the water amount between the upper and lower humidification rollers.
However, as a result of researches, the present inventors found that movement of the water from the upper humidification roller to the lower humidification roller is caused at the timing when the upper and lower humidification rollers make contact with each other. In addition, it was found that the difference in water amount between the upper and lower humidification rollers results in difference in humidification amount between the upper and lower surfaces of the sheet, thus causing sheet curl.
In addition, Japanese Patent Application Laid-Open No. 2012-024953 discloses a mechanism for adjusting the humidification amount by humidification rollers disposed on left and right sides (horizontal positions) during vertical conveyance of sheets. Such a mechanism can function as it is when the humidification rollers are disposed at the same height in the horizontal direction, but cannot be applied to the humidification rollers disposed on the upper and lower sides for conveying sheets in the horizontal direction.
While the movement of the water between a pair of humidification rollers disposed on the upper and lower sides for horizontally conveying sheets is mentioned in the above-mentioned description, the water also moves between the humidification rollers even in the case where a vertical height difference exists between the humidification rollers, for example, in the case where sheets are conveyed obliquely upward or oblique downward. It is to be noted that, even when humidification members having forms other than roller forms are employed, the above-mentioned problems are caused at a pair of humidification members configured to humidify a sheet while sandwiching the sheet.